In the drive for higher integration and operating speeds in LSI devices, the pattern rule is made drastically finer. Under the miniaturizing trend, the lithography has achieved formation of finer patterns by using a light source with a shorter wavelength and by a choice of a proper resist composition for the shorter wavelength. Predominant among others are positive resist compositions which are used as a single layer. These single layer positive resist compositions are based on resins possessing a skeleton having resistance to etching with chlorine or fluorine gas plasma and provided with a resist mechanism that exposed areas become dissolvable. Typically, the resist composition is coated on a substrate to be processed and exposed to a pattern of light, after which the exposed areas of the resist coating are dissolved to form a pattern. Then, the substrate can be processed by etching with the remaining resist pattern serving as an etching mask.
In an attempt to achieve a finer feature size, i.e., to reduce the pattern width with the thickness of a resist coating kept unchanged, the resist coating becomes low in resolution performance. If the resist coating is developed with a liquid developer to form a pattern, the so-called “aspect ratio” (depth/width) of the resist pattern becomes too high, resulting in pattern collapse. For this reason, the miniaturization is accompanied by a thickness reduction of the resist coating (thinner coating). On the other hand, with the progress of the exposure wavelength toward a shorter wavelength, the resin in resist compositions is required to have less light absorption at the exposure wavelength. In response to changes from i-line to KrF and to ArF, the resin has made a transition from novolak resins to polyhydroxystyrene and to acrylic resins. Actually, the etching rate under the above-indicated etching conditions has been accelerated. This suggests the inevitableness that a substrate to be processed is etched through a thinner resist coating having weaker etching resistance. It is urgently required to endow the resist coating with etching resistance.
Meanwhile, a process known as multilayer resist process was developed in the art for processing a substrate by etching. The process uses a resist coating which has weak etching resistance under the etching conditions for the substrate, but is capable of forming a finer pattern, and an intermediate coating which has resistance to etching for processing the substrate and can be patterned under the conditions to which the resist coating is resistant. Once the resist pattern is transferred to the intermediate coating, the substrate is processed by etching through the pattern-transferred intermediate coating as an etching mask. A typical process uses a silicon-containing resin as the resist composition and an aromatic resin as the intermediate coating. In this process, after a pattern is formed in the silicon-containing resin, oxygen-reactive ion etching is carried out. Then the silicon-containing resin is converted to silicon oxide having high resistance to oxygen plasma etching, and at the same time, the aromatic resin is readily etched away where the etching mask of silicon oxide is absent, whereby the pattern of the silicon-containing resin is transferred to the aromatic resin layer. Unlike the single layer resist coating, the aromatic resin need not have light transmittance at all, allowing for use of a wide variety of aromatic resins having high resistance to etching with fluorine or chlorine gas plasma. Using the aromatic resin as the etching mask, the substrate to be processed can be etched with fluorine or chlorine gas plasma.
With respect to the bilayer resist process, active studies were made on the exposure to the radiation (193 nm) of ArF excimer laser and radiation of shorter wavelength where aromatic resins can be no longer used, and several reports have already been made. For example, JP-A 10-324748 and JP-A 11-302382 disclose a siloxane polymer having carboxyl group-containing, non-aromatic monocyclic or polycyclic hydrocarbon groups or bridged cyclic hydrocarbon groups on side chains wherein at least some of the carboxyl groups are substituted with acid labile groups, for example, a siloxane polymer in which a norbornyl group having a t-butoxycarbonyl group at 5-position is bonded to a silicon atom, and a resist composition comprising the polymer. Allegedly this resist composition is less absorptive to KrF (248 nm) excimer laser or ArF excimer laser radiation, forms a pattern of good profile, and is improved in sensitivity, resolution and dry etching resistance. Also, JP-A 2002-055346 and JP-A 2002-268227 disclose that silicone-containing polymers having fluorinated alcohol incorporated therein are less absorptive at the wavelength (157 nm) of F2 laser and improved in sensitivity, resolution and plasma etching resistance.
For the technology intended to improve resolution by reducing the wavelength of an exposure light source, there have been reported compositions using fluorinated siloxane polymers having less absorption at the exposure wavelength of F2 laser. For example, JP-A 2002-220471 discloses that a radiation-sensitive resin composition comprising a polysiloxane having a specific acid-assisted leaving group linked to a silicon atom via at least two bicyclo[2.2.1]heptane rings is useful in that it is improved in dry etching resistance and highly transparent to the radiation of F2 laser. With respect to the technique of improving resolution by forming a thinner coating of material, JP-A 2001-215714 discloses that a silicon-containing polymer having a viscosity in a specific range enables to form a thinner resist coating while maintaining in-plane uniformity in the resist coating.
As the pattern is reduced in feature size, the resist film becomes thinner. For silicone-containing resist compositions allowing film thickness to be reduced, only a limited range of acid generators can be selected to comply with the post-ArF progress toward shorter wavelength. Although a wide variety of acid generators are disclosed in the above-referred patents, the acid generators which are used in post-ArF resist compositions are generally onium salts, and in fact, most examples reported therein use sulfonium salts and iodonium salts. One of the reasons why onium salts are preferentially employed in post-ArF resist compositions is their sensitivity. If organic acid generators other than onium salts are compounded in post-ArF resist compositions, no appropriate sensitivity is achievable. This causes the lithographic process of pattern formation to require an increased exposure dose, resulting in reduced throughputs. The extremely low sensitivity makes it impossible to resolve the pattern to the desired fine feature size.
However, when onium salts are used in silicone based resist compositions as an acid generator, the resist compositions tend to form a pattern with a T-top profile that the top of the pattern becomes extended or overhung because the onium salts have an outstanding dissolution inhibitory effect. Particularly when a silicone-containing polymer having fluorinated alcohol introduced therein is used and an onium salt is used as an acid generator, a T-top profile is often observable. The pattern to be formed by lithography should most preferably maintain rectangularity whereas the T-top profile leads to a failure to form the desired pattern in the etching step following the development. Also, the T-top profile degrades the depth of focus (DOF) in the lithographic process and compromises the minimum resolution ability. For these reasons, the T-top profile should be avoided.
Also on use of onium salts as an acid generator, yet due to the outstanding dissolution inhibitory effect, patterning on a substrate having a high reflectivity results in a pattern profile having standing waves. The generation of standing waves should also be avoided because they can adversely affect the control of pattern size.
Accordingly, in connection with silicone-containing resist compositions which are suited for film thickness reduction, there is an urgent need for a composition which forms a pattern profile with high rectangularity, avoids any loss of dimensional control by the generation of standing waves in a pattern, and has an ability to form a smooth profile.
Among the recent studies on single layer resist coatings where attempts were made to form a finer pattern, it was reported as the cause of pattern collapse that the polymer undergoes substantial swelling immediately before dissolution during development, which inhibits formation of a finer pattern. One effective measure for preventing such swell is to introduce a unit containing a hydroxyl group having an adequately increased acidity due to fluorine substitution at proximate positions, into a resin as a polar group. See H. Ito et al., Journal of Photopolymer Science and Technology, Vol. 15, No. 4 (2002), 591-602. The pattern collapse is a common problem to a silicon-containing resist composition comprising a silicone resin as a base polymer. There is a possibility that the high resolution of the aforementioned polysiloxane polymer be accompanied by this effect.
However, in an actual practice, an attempt to transfer a pattern to an aromatic resin organic coating using a fluorine-rich resin as an etching mask revealed that its resistance to oxygen-reactive etching is far below the expectation. There is a need for further improvement in resistance to etching under these conditions.